How do you change a door mirror on a 1990 Toyota Corolla?

Well if I am correct u have to take inside panel of door off and then there are four brackets that pin it to the doors frame then you go from there. hope I helped you and yes its alot of work

You remove the Philips screw on the triangular plastic plate that covers the adjustment knob or handle that is used to adjust the mirror. After removing the screw you gently pull on the plastic triangular piece and it should come off. There will be a few nuts exposed now that need to be removed and the mirror should then come off. Install mirror in reverse order.